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Purpose: To study the impact of oocyte diameter and cumulus cell mass on the potential for final maturation of immature human oocytes in vitro. Methods: Immature oocytes (n = 1563) from 75 women undergoing fertility preservation by ovarian tissue cryopreservation (14–41 years) were collected. After preparation of the ovarian cortex for freezing, immature oocytes were collected from the surplus medulla. After collection, IVM was performed according to standard published methods. The mass of cumulus cell surrounding the immature oocyte was grouped according to size. After IVM, each oocyte was photographed, measured, and the diameter was calculated as a mean of two perpendicular measurements. Results: The diameter of the oocytes ranged from 60 to 171 µm with a mean of 115 µm (SD:12.1) and an interquartile range from 107 to 124 µm. The oocyte diameter was positively associated with a higher incidence of MII (p < 0.001). MII oocytes had a significantly larger mean diameter than MI, GV, and degenerated oocytes. The size of the cumulus cell mass was significantly associated with the MII stage (p < 0.001) and larger oocyte diameter (p < 0.001). The results further confirm that the diameter of the fully grown oocyte is reached relatively early in human follicular development and that the factors governing oocyte maturation in vitro are connected to the surrounding cell mass and the oocyte. Conclusion: The diameter of the oocyte is a highly determining factor in the nuclear maturation of the human oocyte during in vitro maturation, and the size of the cumulus cell mass is closely positively associated with a larger diameter. [ABSTRACT FROM AUTHOR]